The purpose of this investigation is to describe the host factors involved in the pathogenesis of life threatening bronchiolitis of infants caused by infections with respiratory syncytial (RS) virus or parainfluenza virus type 3. Identification of these factors may lead to development of a rational method of control of lower respiratory disease caused by these agents, which are the most important causes of lower respiratory disease in young children. Both are responsible for serious disease which occurs in infants under 6 months of age and it has been suggested that maternal antibody may participate in the pathogenesis of RS virus disease. Conflicting hypotheses have been presented to explain the disease mechanism. The proposed investigation consists of interrelated studies of patients, animal models and organ cultures. Clinical studies will correlate the initial level of maternal antibody with the severity of disease. Selected patients will be studied for parameters of the immune response including serum and surface antibodies, complement depletion and cellular immunity. The effect of homotypic passively-acquired antibody and prior immunization with aqueous or alum precipitated vaccines will be studied in a hamster model of interstitial pneumonia induced by para-influenza type 3 infection. Human tracheal organ cultures infected with RS virus and parainfluenza virus type 3 will be examined for effect on ciliary function, and for localization of antigen by immunofluorescence and electron microscopy. Definition of the dynamic relationships of host responses to infection in natural and experimental disease may suggest new approaches for prevention or management of life-threatening illnesses in infancy.